1. Field of the Invention
The present invention relates to an audio signal amplitude adjusting device and method for automatically adjusting the amplitude of an input audio signal so as to be within a predetermined range and outputting the resultant signal.
2. Description of the Related Art
Presently, in recording apparatuses for recording audio signals from a microphone collecting voices, or apparatuses for performing various signal processing on audio signals, if the input audio signal is an analog audio signal, this signal is once converted by an A/D converter into a digital audio signal and then taken in. In this operation, if an analog audio signal having an amplitude beyond its input permissible range is input to the A/D converter, it is difficult to faithfully convert the signal into a digital audio signal.
Accordingly, an automatic amplitude control device has been proposed in which a variable attenuator for adjusting the amplitude of an input analog audio signal to be within the input permissible range of the A/D converter is provided at the stage in front of the A/D converter. Refer to, e.g., Japanese Patent Kokai No. H06-314942 (Patent Literature 1).
This automatic amplitude control device obtains the attenuation amount of the variable attenuator based on the level of the input analog audio signal and adjusts the amplitude of the analog audio signal by the attenuation amount. For example, when an analog audio signal having an amplitude over the upper limit of the input permissible range of the A/D converter is input, the attenuation amount is increased, thereby reducing the amplitude of the analog audio signal to be within the input permissible range, and thereafter when the amplitude of the input analog audio signal itself has shifted to within the input permissible range, the attenuation amount increased as mentioned above is reduced.
In the automatic amplitude control device, for the amplitude adjustment as described above, the response period from the increase in the attenuation amount to the audio signal amplitude going inside the input permissible range (called an attack time period) is made short. And the response period from when later the amplitude of the input audio signal itself has shifted to within the input permissible range to when the attenuation amount has been reduced to an original attenuation amount (called a recovery time period) is intentionally made long, thus avoiding the amplitude-adjusted audio signal becoming unnatural for the auditory sense.